This invention relates to the marking for identification purposes of sheet articles of non-conductive material, particularly such articles which are in paper sheet form e.g. banknotes, passports and bonds.
One method of marking articles of paper sheet material so that the articles can be identified and their authenticity thereby checked involves the incorporation herein of a detectable material which however must not alter too much the appearance and properties of the article. The proportion of detectable material incorporated into the articles must therefore in general be small. Furthermore, it is generally desirable that the detection system by very sensitive, that it be capable of rapid response in order to allow identification of the article at high speeds, and that it should provide a reliable means for repeated identifications of the same articles. Finally, it is also desirable that the detectable material be capable of producing a specific response, which can difficultly be imitated by other materials, in order to avoid successful counterfeiting of the markings.